Gas analyser

ABSTRACT

Gas analyzer comprising a measuring chamber ( 3 ), a gas inlet channel ( 1 ), a gas outlet channel ( 14 ), a pump ( 13 ) for providing a gas flow through the measuring chamber ( 3 ), a device for irradiating the gas in the measuring chamber ( 3 ) with pulsating electromagnetic energy in form of infrared light so as to generate acoustic pressure fluctuations therein, and at least one microphone ( 9 ) communicating with the measuring chamber for detecting said pressure fluctuations. The inlet channel ( 1 ) and the outlet channel ( 14 ) each comprises an acoustic filter ( 2,11 ) in form of channel sections with reduced sectional area of flow (R 1 -R 9 ) and a plurality of cavities (C 1 -C 7 ) associated therewith. The gas analyzer comprises at least one sandwich device formed of joined plate-like elements, the channel sections with reduced flow area (R 1 -R 9 ) of the inlet channel ( 1 ) and/or the outlet channel ( 14 ) being defined by at least two elements and the vibration-reducing cavities (C 1 -C 7 ) also being defined by at least two elements, the connections between the respective channel and the respective cavities being provided as transverse apertures in the elements or element portions adjoining the channel and the cavities, respectively.

TECHNICAL FIELD

[0001] The invention relates to a gas analyser comprising a measuringchamber, a gas inlet channel for supplying gas to the measuring chamber,a gas outlet channel for removing gas from the measuring chamber, meansfor providing a predetermined volume flow rate through the measuringchamber, a device for affecting the gas in the measuring chamber withpulsating magnetic or electromagnetic energy of predetermined pulserates so as to generate acoustic pressure fluctuations therein, theinlet channel and the outlet channel each comprising an acoustic filterin form of channel sections with reduced sectional area of flow so as toprovide a predetermined flow resistance, and a plurality of cavities ofa predetermined volume associated therewith.

BACKGROUND ART

[0002] The gas analyser of the above type may either be formed as aso-called paramagnetic gas analyser employed for measuring the oxygencontent in a gas or as a so-called photoacoustic gas analyser, whichhereinafter is denoted as PGA (photoacoustic gas analyser), employed formeasuring the incidence of one or more specific gasses in a gas mixture.

[0003] In a paramagnetic electromagnetic gas analyser the affectingdevice is an electromagnet affecting the gas in the chamber with apulsating magnetic field. As oxygen in practice is the only occurringgas, which is paramagnetic, the pulsating magnetic field generatespressure changes in the gas in the measuring chamber depending on theoxygen portion in the gas. These pressure changes are detected by meansof a microphone.

[0004] In a photoacoustic gas analyser is the affecting device is anelectromagnetic radiation source affecting the gas in the measuringchamber with electromagnetic radiation, eg. infrared light. The energyfrom the light source is periodically absorbed by gas mixture and causesa periodic heating resulting in a corresponding increase in the pressureof the gas. The gas is cooled between the light pulses, whereby thepressure in the chamber decreases correspondingly. These pressurechanges are detected by means of a microphone connected to the chamber.Due to the composition of the individual gas mixture constituents theabsorption wavelength thereof differs from one another. If thewavelength of the modulated light is set to be close to one of theseabsorption wavelengths, the rise in temperature and thus the pressure inthe chamber increases as the portion of the gas constituent having theabsorption wavelength in question increases.

[0005] Both in a paramagnetic and in a photoacoustic gas analyser theaccuracy of the measurement is conditional on the measuring chamberbeing kept closed during the measuring process in the sense that theproduced pressure fluctuations are retained in the chamber and externalnoise is prevented from entering the chamber. For instance in connectionwith a photoacoustic gas analysis this may be obtained by allowing ameasured amount of gas to enter the measuring chamber prior to ameasurement and subsequently closing off the measuring chamber duringthe measurement per se.

[0006] This measuring method is known from U.S. Pat. No. 4,818,882 andused in the device entitled Multi-gas Monitor, type 1302, from thecompany of Innova Air Tech Instruments. The said measuring method is,however, encumbered by the drawback that the measurement is intermittentand the measuring time is relatively long, typically of 30-60 seconds.As some physiological measurements require a response time of typically0.1 second, this measuring method is unsuitable for such applications.Another measuring method is to employ so-called acoustic filters toprevent noise from reaching the measuring chamber and affecting themeasuring results—at least in the frequency area corresponding to thephotoacoustic frequencies. Such acoustic filters allow for a constantvolume flow rate through the measuring chamber and is described in theabove U.S. Pat. No. 4,818,882.

[0007] The said acoustic filters may be formed of narrow flow channelsor restrictions providing resistance to gas flow therethrough and ofcavities communicating therewith attenuating pressure changes in the gasflow. In order to facilitate the calculation of acoustic filters, therestrictions and the cavities can be equivalent to the electricresistances and capacities, respectively, as the differential equationsused for the two systems also are equivalent. Pressure and volume flowrate thus correspond to electric voltage and current, respectively. Thecavities may communicate with the flow channels such that the gas flowsthrough said cavities. If, however, a quick response time is required,it is advantageous in relation to the inlet channel to connect thecavities to the flow channel via lateral branches such that theinflowing gas does not flow through the cavities and thereby is mixedwith the gas therein.

[0008] The number and size of the restrictions and the cavities isdetermined by the desired physical size of the system and of thefrequencies to be dampened by the acoustic filter. Long, thinrestrictions offer more resistance to flow than short, wide restrictionsand large cavities dampen lower frequencies than small cavities. Forobtaining sufficient damping, an acoustic filter may comprise severalsuccessive parts formed of a restriction and a cavity, respectively.

[0009] The device known as Anaesthetic Gas Monitor Type 1304 from thecompany of Innova Air Tech Instruments, discloses a PGA comprisingacoustic filters of the above type, in which the restrictions of theacoustic filters are formed of thin needle tubes and each cavity of ametal container with a hose connector communicating with the cavity ofthe container. The needle tubes and the container connectors areinterconnected by means of a short silicone hose. One drawback of such astructure is that it is relatively sensitive to vibrations. Yet anotherdrawback is that the structure comprises a large number of componentsresulting in an expensive and time-consuming manufacture and assemblingthereof. Furthermore, in needle tubes with a circular inner crosssection, the flow resistance is inversely proportional to the radius inthe fourth power. Consequently deviations from the nominal radius resultin a quadruple relative deviation of the flow resistance from thenominal value thereof.

[0010] A need thus exists for a gas analyser with acoustic filters,which is less sensitive to vibrations and which is more simple tomanufacture than the known gas analysers.

BRIEF DESCRIPTION OF THE INVENTION

[0011] The gas analyser according to the invention is characterised inthat it comprises at least one sandwich device formed of joinedplate-like elements, the channel sections with reduced flow area of theinlet channel and/or the outlet channel being defined by at least twoelements and the cavities also being defined by at least two elements,the connections between the respective channel and the respectivecavities being provided as transverse apertures in the elements orelement portions adjoining the channel and the cavities, respectively.

[0012] Since the sandwich device is a mechanically rigid structure,which is insensitive to vibrations, a photoacoustic gas analyser, whichis less sensitive to vibrations than known gas analysers, is obtained.Furthermore it is comparatively simple to manufacture and assemble therelatively few elements of the sandwich device. In addition to theacoustic filter it is, moreover, also possible to incorporate other flowchannels of the analyser into the sandwich device. The sandwich devicethus enables the provision of a very compact gas analyser. Furthermorethe fundamental structure of the sandwich device, wherein flow channelsare arranged at different levels, is comparable with the structure ofprinted circuit boards.

[0013] According to a preferred embodiment of the invention, theacoustic filters of the inlet channel and the outlet channel may beprovided in the same sandwich device so as to obtain a simple andcompact embodiment.

[0014] Moreover according to the invention the channel or channels ofthe sandwich device may be defined by a non-through-going groove in afirst element and an adjoining second element, in which the transverseapertures connecting the channel with the cavities is formed.

[0015] Furthermore according to the invention the channel or channels ofthe sandwich device may be defined by a through-going slit in a firstelement and two further adjacent elements, one provided on either sideof the first element, the transverse apertures connecting the channelwith the cavities being formed in one of the further elements.

[0016] According to a preferred embodiment of the invention the firstelement may be formed of a metal foil having a thickness of between 0.05and 0.5 mm, preferably 0.1 and 0.2 mm, the width of the slit exceedingthe foil thickness, preferably by at least two to three times. As themetal foil thus preferably is rather thin compared to the width of theslit, the width of the channel somewhat exceeds its height, whereby adeviation from the nominal height of the channel only results in thetriple deviation of the flow resistance from its nominal value. Sincemetal foils of very accurate thickness tolerances are readily available,it is expected that a higher degree of accuracy of the flow resistancemay be obtained by use thereof than by use of needle tubes. The slits inthe metal foils are formed in an accurate processing method, preferablyan etching method, so as to prevent burrs on the element in question.

[0017] Furthermore according to the invention the transverse aperturesmay be formed in the same element as the cavities, the cavities beingdefined by a separate plate-like element on the side opposite thetransverse apertures.

[0018] Moreover according to the invention the cavities may be formed asrecesses in a separate element abutting the elements comprising thetransverse apertures.

[0019] Furthermore according to the invention each of the cavitiesassociated with the acoustic filter of the inlet channel may communicatewith the acoustic filter of the outlet channel via a shunt channel. Theshunt channel provides a minor partial flow through the cavitiesassociated with the acoustic filter of the inlet channel and bypassed ofthe measuring chamber to ensure that a gas interchange between the gasin the inlet channel and the gas in the associated cavities is noteffected. Such a gas interchange would have an adverse effect on theresponse time and the measuring accuracy. The shunt channel is typicallydimensioned such that the volume flow rate therethrough is in the orderone tenth of the volume flow rate through the measuring chamber.

[0020] Furthermore according to the invention the shunt channel may beprovided partly by means of at least two plate-like elements of thesandwich device defining shunt channel sections, said elementspreferably differing from the elements defining the inlet channel, theoutlet channel and the cavities, respectively, and partly by transverseapertures connecting the shunt channel sections with the cavities in theacoustic filter of the inlet channel and with the acoustic filter of theoutlet channel, respectively.

[0021] Finally according to the invention substantially all of the flowchannels of the gas analyser may be provided in elements of the sandwichdevice. As a result a very compact and rigid structure is obtained whichis insensitive to vibrations.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The invention is explained in detail below with reference to thedrawings, in which

[0023]FIG. 1 is a flow chart of a photoacoustic gas analyser accordingto the invention,

[0024]FIG. 2 is a chart of the gas flow in the gas analyser,

[0025]FIG. 3 is a perspective illustration of a preferred embodiment ofa gas analyser according to the invention,

[0026]FIG. 4 is an exploded view of the gas analyser shown in FIG. 3,

[0027]FIG. 5 illustrates plate-shaped elements forming part of thesandwich device of the gas analyser shown in FIGS. 3 and 4, saidelements forming the acoustic filters of the inlet and outlet channels,and

[0028]FIG. 6 illustrates further plate-shaped elements forming part ofthe sandwich device, said elements forming a shunt channel.

BEST MODE FOR CARRYING OUT THE INVENTION

[0029] The photoacoustic gas analyser shown in FIG. 1 comprises a gasinlet channel 1 provided with a first acoustic filter 2 and throughwhich the gas to be analysed is passed to inlet of a photoacousticmeasuring chamber 3. Light is emitted towards the measuring chamber 3 bymeans of an infrared light source 5 (other types of electromagneticradiation may also be used) and a reflector 15. Before reaching themeasuring chamber the light passes a so-called chopper wheel 6comprising at least one—and in the present case three—concentric rows ofapertures. The apertures in each row are of the same size and mutuallyevenly interspaced, while the number of apertures in the rows differsfrom each other corresponding to the different modulating frequencies.The wheel is made to rotate about its centre by means of a motor 7,whose rotational speed is controlled by means of an optical tachometer8. The chopper wheel 6 generates the pulsating electromagnetic radiationnecessary for generating measurable pressure fluctuations in the gasconstituents to be detected in the measuring chamber 3 by means of thegas analyser. The electromagnetic radiation pulsed by the chopper wheel6 then passes a number of optical filters 4 corresponding to the numberof rows of apertures. Each of the optical filters selects a specificwavelength of the infrared light corresponding to the absorption bandsof each of the gasses to the measured. Then the light passes through afirst window 17, which is transparent to infrared light, and into themeasuring chamber and having passed therethrough the light exits throughanother window 18, which also is transparent to infrared light. Themeasuring chamber 3 further communicates with a gas outlet channel 14provided with a pump 13. A second acoustic filter 11 is provided in thegas outlet channel 14 between the pump 13 and the measuring chamber 3. Ameasuring microphone 9 communicates with the gas outlet channel 14between the measuring chamber 3 and the second acoustic filter 11, saidmicrophone 9 detecting the acoustic signals generated in the measuringchamber 3 and converting these into an electric signal amplified by amicrophone amplifier 10 for further signal processing.

[0030] The gas flows in the photoacoustic gas analyser in FIG. 1 and inparticular the structure of the first filter 2 and the second filter 11associated with the inlet channel 1 and the outlet channel 14,respectively, is shown in FIG. 2. The first acoustic filter 2 comprisesa number of inlet channel sections with reduced sectional area of flowor restrictions R1,R2,R3,R4,R5 and a plurality of cavities C1,C2,C3 andC4 communicating with separate lateral branches between the restrictionsfor obtaining a fast response time. The second acoustic filter 11 of thegas outlet channel 14 similarly comprises a number of channel sectionswith reduced flow area or restrictions R6,R7,R8 and R9 and cavitiesC5,C6 and C7 connected therewith. The restrictions R6 to R9 of theoutlet channel communicate with the cavities C5 to C7 such that a gasflow through the cavities is provided. This rather simple embodiment ofthe acoustic filter may be used in the outlet channel 14 withoutadversely affecting the response time.

[0031] By means of a shunt channel 12 comprising channel sections withreduced flow area or restrictions RS2, RS3, RS4 and RS5 each of thecavities C1, C2, C3 and C4 communicates with the outlet channel 14 inthe area at the cavity C5 thereof. The shunt channel 12 serves toprovide a shunt flow through the cavities C1 to C4 and bypassed of themeasuring chamber 3. This shunt flow prevents a gas interchange betweenthe gas in the cavities C1-C4 and the gas flow in the inlet channel 1and thus improves the response time and the accuracy of measurement.Finally it appears from FIG. 2 that the microphone 9 communicates withthe outlet channel 14 immediately downstream of the measuring chamber 3by means of a microphone channel 19. It should be noted that themicrophone 9 of course may communicate directly with the measuringchamber 3 via a separate channel.

[0032]FIGS. 3 and 4 show a preferred embodiment of a gas analyseraccording to the invention comprising a frame 20, on which a sandwichdevice 21 is arranged comprising a plurality of superposed and joinedplate-shaped elements 22-30 for providing the flow channels and acousticfilters of the gas analyser. When seen in direction from the frame 20,the sandwich device 21 comprises a block 22 in which the cavities C1-C7are provided in form of recesses. A gasket 23 is sandwiched between theblock 22 and a lower plate 24, said plate upwardly defining the cavitiesand being provided with transverse apertures for connecting therespective cavities with a superposed main channel foil 25. The mainchannel foil 25 is provided with through-going slits so as to form interalia the inlet channel 1 and the outlet channel 14. Downwardly, thelower plate 24 defines the slits in the main channel foil 25, while theslits are upwardly defined by a separation foil 26. The separation foil26 is provided with through-going apertures to provide the necessaryflow connections between the plate-shaped element arranged thereunderand a shunt channel foil 27 arranged thereabove and defining the shuntchannel 12. The slit formed in the shunt channel foil is downwardlydefined by the separation foil 26 and upwardly defined by a cover foil28 so as to form the shunt channel 12. A cover plate 30 is arrangedabove the cover foil 28, a pressure-distributing gasket 29 beingarranged therebetween. The plate-shaped elements 23-29 arranged betweenthe cover plate 30 and the block 22 are bolted together between theblock and the cover plate by means of bolts (not shown), said boltsextending through apertures 31 in the elements and screwed into threadedholes 35 in the block 22.

[0033] As regards the sandwich device 21 it should be noted that thegaskets 23 and 29 optionally may be omitted and that the cavities andthe restriction thereof may be provided in manner differing from themanner described above. By omitting the gasket it is thus possible alsoto form the lower plate 24 integrally with the block. The cavities arethen provided by means of recesses in the lower face of the block anddownwardly defined by a separate plate-shaped element.

[0034] A housing 32 is mounted on the cover plate 30 as retainer for themeasuring chamber 3 and the optical filters 4. The motor 7 for drivingthe chopper wheel 6 is attached to the frame 20 in the space between thelegs of the sandwich device 21, which is U-shaped in a top view. Theinfrared light source 5 with associated reflector 15 is arranged in asupport 33 attached to the cover plate 30. The microphone 9 (not shownin FIGS. 3 and 4) is arranged in a microphone housing 34 received in arecess on the lower face of the block. Finally it should be noted thatthe inlet opening 38 of the gas inlet channel 1 and the outlet opening37 of the gas outlet channel 14 (confer FIGS. 5 and 6) are formed ofthrough-going apertures in the bottom of the block and that a pumpcorresponding to the pump 13 in FIGS. 1 and 2 is connected to the outletof the gas outlet channel.

[0035] The structure of the sandwich device 21 is described in detailbelow with reference to FIGS. 5 and 6. FIG. 5 is a top view of the block22, the cavities C1-C7 formed therein shown by means of solid lines. Thethrough-going slits formed in the main channel foil 25 are shown bymeans of dotted line, said slits defining the inlet channel 1 and theoutlet channel 14 with the associated restrictions R1-R5 and R6-R9,respectively. Furthermore the through-going apertures in the lower plate24 are illustrated by means of dotted circles, said apertures connectingthe cavities with the channels. It should furthermore be noted that thesame reference numerals as used in FIG. 2 have been used for thecavities and the restrictions of the channels. The gas to be analysedflows from below into the block through the inlet 38 and thus enters theinlet channel 1, which is defined by the slit in the main channel foil25 and the lower plate 24 and the separation foil 26 arranged on eitherside of the main channel foil 25. The inlet channel comprises therestrictions R1,R2,R3,R4 and R5. The inlet channel 1 communicates withthe chamber Cl between the restrictions R1 and R2 via the aperture C1 ain the lower plate 24. In a corresponding manner between therestrictions R2 and R3 the inlet channel 1 communicates with the cavityC2 via the aperture C2 a in the lower plate 24, between the restrictionsR3 and R4 the channel communicates with the cavity C3 via the apertureC3 a and between the restrictions R4 and R5 the channel communicateswith the cavity C4 via the aperture C4 a. The restriction R5communicates with an aperture C0 a communicating with the inlet to themeasuring chamber 3 in a manner not shown in detail. Correspondingly theoutlet of the measuring chamber 3 communicates with the aperture C0 b ina manner not shown in detail, said aperture communicating with therestriction R6 of the outlet channel 14. As the further restrictions R7,R8 and R9 of the outlet channel, R6 is formed as a through-going slit inthe main channel foil 25. The restriction R6 communicates with thecavity C5 via a through-going aperture C5 a in the lower plate 24.Furthermore the cavities C5 and C6 are interconnected by means of therestriction R7 and the apertures C5 b and C6 c in the lower plate 24. Ina corresponding manner the cavity C6 communicates with the cavity C7 viathe restriction R8 and the apertures C6 a and C7 a in the lower plate24. The cavity C7 is formed of two cavity halves interconnected by meansof a channel 36 in the block 22 and the cavity C7 communicates with theoutlet 37 of the block via the restriction R9 and an aperture C7 b andan aperture arranged in the lower plate 24 above the outlet 37.

[0036]FIG. 6 is a top view in direction of the block, which is shown assolid lines, of the slits formed in the shunt channel foil 27 forproviding the shunt channel 12 with its restrictions RS2-RS5 shown bymeans of dotted lines. Furthermore apertures in the separation foil 26and the subjacent foils extending up to the cavities C1-C5 in the block22 are illustrated by means of circles. The same reference numerals asin FIG. 2 have also been used in FIG. 6. The restriction RS2 is definedby a slit in the shunt channel foil 27, the subjacent separation foil 26and the superjacent cover foil 28, said restriction furthercommunicating with the cavities C1 and C2 via the apertures C1 c and C2b, respectively. Correspondingly the slit forming the restriction RS3 inthe shunt channel foil 27 communicates with the cavities C2 and C3 viathe apertures C2 c and C3 b, the slit forming the restriction RS4 in theshunt channel foil 27 communicates with the cavities C3 and C4 via theapertures C3 c and C4 b, respectively, and the slit forming therestriction RS5 in the separation foil 26 communicates with the cavitiesC4 and C5 via the apertures C4 c and C5 c, respectively. It should benoted that some flow channels and through-going apertures defined by theelements of the sandwich device 21 have been omitted in FIGS. 5 and 6 inconsideration of clarity. Moreover some flow channels and ancillaryequipment have been omitted in FIGS. 1 and 2 for the same reason.

[0037] The invention has been described above with reference to aphotoacoustic gas analyser comprising a sandwich device forming theacoustic filters. It should, however, be understood that the inventionis not limited to the described embodiment or to photoacoustic gasanalysers, but also comprises paramagnetic gas analysers within thescope of protection defined by the claims.

1. Gas analyser comprising a measuring chamber, a gas inlet channel (1)for supplying gas to the measuring chamber (3), a gas outlet channel(14) for removing gas from the measuring chamber, means (13) forproviding a predetermined gas volume flow rate through the measuringchamber (3), a device (5,15,6,7) for affecting the gas in the measuringchamber (3) with pulsating magnetic or electromagnetic energy ofpredetermined pulse rates so as to generate acoustic pressurefluctuations therein, and at least one microphone (9) communicating withthe measuring chamber for detecting said acoustic pressure fluctuations,the inlet channel and the outlet channel each comprising an acousticfilter (2,11) in form of channel sections with reduced sectional area offlow (R1-R9) so as to provide a predetermined flow resistance and aplurality of cavities (C1-C7) of a predetermined volume associatedtherewith, characterised in that the gas analyser comprises at least onesandwich device (21) formed of joined plate-like elements (22-30), thechannel sections with reduced flow area (R1-R9) of the inlet channel (1)and/or the outlet channel (14) being defined by at least two elements(24,25,26) and the cavities (C1-C7) also being defined by at least twoelements (22,23,24), the connections between the respective channel andthe respective cavities being provided as transverse apertures in theelements or element portions adjoining the channel and the cavities,respectively.
 2. Gas analyser according to claim 1, characterised inthat the acoustic filters (2,11) of the inlet channel and outlet channel(1,14), respectively, are provided in the same sandwich device (21). 3.Gas analyser according to claim 1 or 2, characterised in that thechannel or channels of the sandwich device (21) is/are defined by anon-through-going groove in a first element and by an adjoining secondelement, in which the transverse apertures connecting the channel withthe cavities are formed.
 4. Gas analyser according to claim 1 or 2,characterised in that the channel or channels of the sandwich device(21) is/are defined by a through-going slit in a first element (25) andby adjoining further elements (24,26), one provided on either side ofthe first element, the transverse apertures connecting the channel withthe cavities being formed in one of the further elements (24).
 5. Gasanalyser according to claim 4, characterised in that first element (25)is formed of a metal foil having a thickness of between 0.05 and 0.5 mm,preferably 0.1 and 0.2 mm, the width of the slit exceeding the foilthickness, preferably by at least two to three times.
 6. Gas analyseraccording to claim 3 or 4, characterised in that the transverseapertures are formed in the same element as the cavities, the cavitiesbeing defined by a separate plate-like element on the side opposite thetransverse apertures.
 7. Gas analyser according to claim 3 or 4,characterised in that the cavities (C1-C7) are formed as recesses in aseparate element (22) abutting the element (24) comprising thetransverse apertures.
 8. Gas analyser according to claim 1,characterised in that each of the cavities (C1-C5) of the acousticfilter (2) of the inlet channel (1) communicates with the acousticfilter (11) of the outlet channel (14) via a shunt channel (12).
 9. Gasanalyser according to claim 8, characterised in that the shunt channel(12) is provided partly by means of at least two plate-like elements(26,27,28) of the sandwich device (21) defining shunt channel sections,said elements preferably differing from the elements defining the inletchannel, the outlet channel and the cavities, respectively, and partlyby means of transverse apertures connecting the shunt channel sections(RS2-RS5) with the cavities in the acoustic filter (2) of the inletchannel and with the acoustic filter (11) of the outlet channel (14),respectively.
 10. Gas analyser according to one or more of the precedingclaims, characterised in that substantially all of the flow channelsforming part of the gas analyser are provided in elements of thesandwich device (21).